/*
 * OREKIT-X
 * Copyright 2002-2008 CS Communication & Systemes
 * 
 * Parts of this software package have been licensed to CS
 * Communication & Systemes (CS) under one or more contributor license
 * agreements.  See the NOTICE file distributed with this work for
 * additional information.
 *  
 * This is an experimental copy of OREKIT from www.orekit.org.
 * Please use the original OREKIT from orekit.org for normal work
 * unrelated to this research project.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you
 * may not use this file except in compliance with the License.  You
 * may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
 * implied.  See the License for the specific language governing
 * permissions and limitations under the License.
 */
package ore.propagation.events;

import ore.Frame;
import ore.PositionVelocity;
import ore.SpacecraftState;
import ore.bodies.BodyShape;
import ore.bodies.GeodeticPoint;
import ore.errors.OrekitException;

/** 
 * Finder for satellite altitude crossing events.
 * 
 * This class finds altitude events (i.e. satellite crossing a
 * predefined altitude level above ground).
 * 
 * The default implementation behavior is to continue propagation when
 * ascending and to stop propagation when descending. This can be
 * changed by overriding the eventOccurred method in a derived class.
 * 
 * @author Luc Maisonobe
 */
public class AltitudeDetector 
    extends AbstractDetector
{
    /** 
     * Threshold altitude value.
     */
    private final double altitude;
    /** 
     * Body shape with respect to which altitude should be evaluated.
     */
    private final BodyShape bodyShape;


    /** 
     * The maximal interval between elevation checks should be smaller
     * than the half period of the orbit.
     * 
     * @param maxCheck maximal interval in seconds
     * @param altitude threshold altitude value
     * @param bodyShape body shape with respect to which altitude should be evaluated
     */
    public AltitudeDetector(double maxCheck, double altitude, BodyShape bodyShape){
        super(maxCheck, 1.0e-3);
        this.altitude = altitude;
        this.bodyShape = bodyShape;
    }


    /** 
     * @return The threshold altitude value
     */
    public double getAltitude() {
        return this.altitude;
    }
    /** 
     * @return The body shape
     */
    public BodyShape getBodyShape() {
        return this.bodyShape;
    }
    /** 
     * Handle an altitude event and choose what to do next.
     * 
     * The default implementation behavior is to {@link
     * EventDetector#CONTINUE continue} propagation when ascending and
     * to {@link EventDetector#STOP stop} propagation when
     * descending. This can be changed by overriding this method in a
     * derived class.
     * 
     * @param s The current state information : date, kinematics, attitude
     * @param increasing When true, the value of the switching
     * function increases when times increases around event (note that
     * increase is measured with respect to physical time, not with
     * respect to propagation which may go backward in time)
     * 
     * @return One of {@link #STOP}, {@link #RESET_STATE}, {@link #RESET_DERIVATIVES}
     * or {@link #CONTINUE}
     * 
     * @exception OrekitException Some specific error occurs
     */
    public int eventOccurred(SpacecraftState s, boolean increasing)
        throws OrekitException
    {
        return increasing ? CONTINUE : STOP;
    }
    /**
     * {@inheritDoc}
     */
    public double g(final SpacecraftState s)
        throws OrekitException
    {
        final Frame bodyFrame         = bodyShape.getBodyFrame();
        final PositionVelocity pvBody = s.getPositionVelocity(bodyFrame);
        final GeodeticPoint point     = bodyShape.transform(pvBody.getPosition(),
                                                            bodyFrame, s.getDate());
        return (point.getAltitude() - altitude);
    }
}
